The present invention relates to a blanking method and device in a plate elements cutting machine, for example, for cutting paper, cardboard or plastic sheets.
Manufacture of such sheets is generally executed within a production line mainly made up of a cutting station, a waste stripping station and a blanking and delivery station. Sheets are brought successively from one station to the other by a gripper bar, mounted on chain members, that seize the front side of the sheet. At each station, the chain members are stopped and each sheet is liable to a transformation constituting a new stage in the manufacturing process.
It is possible to produce, for example, packing boxes, labels and any other articles that are generally preprinted from each sheet. Developed opened during the manufacturing process, these articles are ordinarily called blanks by professionals.
These sheets are first cyclically introduced one after the other in a platen press, which will cut as many blanks as allowed by the surface capacity of the sheet. In order to avoid the sheet falling into pieces once cut up, all blanks are designed to be held together by points of attachment or small material bridges, sometimes called nicks, obtained by (non cutting) mortises realized in the cutting rules of the press tool.
Despite the planned disposition of the blanks over the surface of the sheet, it is generally not possible to avoid all production of interstitial losses. It is thus necessary to pass each of these sheets to another station, called waste stripping station, in order to withdraw all the undesired parts, by nipping between several stripping members. After this process, the sheet is then brought by a gripper bar to a blanks separating and delivering station, where these blanks are precisely unfastened from each other and are carefully piled up on a palette to form as many piles as the number of blanks of a sheet. Finally, the residual sheet, or skeleton, which includes the sheet's front side, is released by the gripper bar in an exit station.
The present invention is used in the blanks separating station of the machine.
An upper mobile tool and a lower fixed tool are generally used in a blank separating machine. The upper tool is comprised of a series of punches and the lower tool is comprised of a board having apertures located face to face with the punches. In a downwards vertical motion, the upper mobile punches press on all blanks at once and into the apertures of the lower tool, thus breaking the points of attachment that linked each blank to the remaining sheet. The separating tools have therefore to fit with the form and disposal of the blanks of each new series of sheets to be manufactured. Generally, the punches are strip aligned with regards to the cutting lines of the press, on a basis plate fixed on an upper tool rack frame of the blank separating machine. On the lower tool, there is a corresponding opening or a mesh under and opposite to each punch. The tool is made up of rods defining the meshes, or of largely perforated board.
Patents CH682651 and EP763407 provide more details of such a device that works very well for big or medium size blanks. But, when it is necessary to separate small size or small with blanks, such as strips, these blanks tend to pivot or to turn over when they are released from the sheet. As a consequence, they cannot be piled up properly and that process becomes uncertain and possibly wrong. When the surfaces of the blanks are important enough, which is generally the case in packing manufacture, these blanks naturally fall floating and piling up correctly on the palette. It is observed that the ratio between the height of fall and the blank surface is small enough to avoid the blanks turning over. But, this height of fall, which is generally around 65 mm high, can hardly be reduced due to the various size constraints, either because of the high density of mechanical pieces in the machine or because it is necessary to provide a defined space between the tools, for example in case of a jam, in order to be able to easily withdraw the damaged sheets.
The problem occurring with wrong piling of blanks is particularly apparent when these blanks are produced from a plastic sheet. Indeed, this kind of material often confers a good elasticity to the blanks while bent. Now, this is precisely the case in the blank separating process. When a punch presses on the blank, these blanks bend more and more until the sudden break up of the attachment points. At that moment, the blank is hurled down at a very high speed under the release effect that is abruptly produced when the material bridges separate. The blank propelling speed is far greater than the punch movement so that the falling of the blank gets completely out of control. It has also been observed that the “spring effect” of the release is more apparent because the material used resists the rupture (as is the case with plastic as compared with cardboard). The depth (around 0.2 to 0.6 mm) and geometry of the blank also increase this reaction especially when blanks are particularly narrow and long and/or the cuts are in small strips form, for example.
Another disadvantage is observed when the attachment points do not break all at the same time while the punch goes through the lower tool. This often arises when the processed blanks are made up of a low depth (typically between 40 μm and 0.1 mm) synthetic substance, such as polypropylene. Thus, if all the attachment points of a same side break up correctly but not the ones on the opposite side, the still attached blanks could not be completely removed and would remain hanging under their sheet by the remaining points of attachment. In this situation, either a jam would occur in the machine or a disarrangement in the piling would occur when the blank eventually falls at the withdrawal time of the remaining sheet.